1. Field of the Invention
The present invention relates to a system for encoding and decoding amplitude modulated signals and, more particularly, to a system for scrambling or encoding transmitted video signals so that the received video display is severely distorted and for unscrambling or decoding the received signal to permit a normal video display.
2. Description of the Prior Art
In the rapidly expanding field of pay cable television, it is common to transmit a number of television programs on frequency channels within the VHF spectrum but which do not coincide with any of the normal channels 2 through 13 of a standard television receiver. Thus, in order for a subscriber to the cable television service to receive these television programs, he is provided with a converter which will convert the frequency channels to one of the normal channels of the television receiver. Typically, such converter is a conventional superhetrodyne tuner including a local oscillator and a mixer, the latter mixing the frequency of the local oscillator with the incoming frequency to convert it to one of the available, but unused channels of the television receiver.
Obviously, the intent of such a system is that only those individuals who subscribe to the cable television service shall have access to a converter which will permit reception of these television programs on a standard TV set. However, because of the relative ease in building a superhetrodyne tuner, which only requires a local oscillator and mixer, a number of converters have been made generally available or have been made individually. Thus, many individuals who have not paid for the service nevertheless have the service provided to them.
In order to overcome this problem, it has been proposed to alter the transmitted signal in a manner which would prevent or severely distort a normal video picture even if the frequency channel was converted to one of the normal channels of the television receiver. Such altering of the transmitted signal is often referred to as scrambling or encoding. Therefore, in addition to requiring a converter, the individual must have suitable apparatus to unscramble or decode the received signal if he is to receive an intelligible video picture.
With this in mind, numerous attempts have been made to provide encoding/decoding systems for cable television systems. However, to date, none of the suggested systems have been acceptable for a variety of reasons. Some proposed systems have required a significant increase in the bandwidth of the transmitted signal and this is generally unacceptable because of the limitation that it places on the number of available channels. Other systems have been highly complex and therefore expensive, making them impractical for a wide-spread cable television system. Still other systems, while working adequately with older, less sophisticated television receivers, do not work effectively with the newer, more sophisticated television sets which tend to be less susceptible to slight or minor distortions of an incoming video signal.